As a rule, known fuel cell systems for operation in an atmosphere in which humans can breath are supplied with air, oxygen and hydrogen. In order to provide the air turnover necessary for complete conversion of the hydrogen, and thus to provide the necessary quantity of oxygen, fans or compressors are used on the air side. As a rule, these systems operate with excess air.
The compressors require energy, e.g. electrical energy, for operation; they emit noise and increase the overall weight of the system.
In addition, compressors comprise movable components that are subject to wear. This may reduce the overall reliability of such a system.
For the disposal of waste water from the passenger region, in today's commercial aircraft so-called vacuum waste-water systems are used. These systems transport sewage with the use of air as a transport medium, which air at cruising altitude flows from the on-board toilet to the waste water tank as a result of the pressure differential between the aircraft environment and the cabin pressure. In this arrangement the waste water tank is directly connected to the exterior air.
At low flight altitudes or on the ground the pressure differential required for the transport of sewage is generated by means of a vacuum generator. In this arrangement the waste water tanks used are, as a rule, constructed as pressure tanks that are designed for operation at a maximum possible pressure differential.